1. Field of the Invention
The present invention relates mainly to steel sheets for automobiles, and more particularly, to high-ductility steel sheets having very high strain age hardenability and excellent press formability such as ductility, stretch-flanging formability, and drawability, in which the tensile strength increases remarkably through a heat treatment after press forming, and to methods for manufacturing the same. The term xe2x80x9csteel sheetsxe2x80x9d as herein used shall include hot-rolled steel sheets, cold-rolled steel sheets, and hot-dip galvanized steel sheets. The term xe2x80x9csteel sheetsxe2x80x9d as herein used shall also include steel sheets and steel strips.
2. Description of the Related Art
In recent years, weight reduction in automobile bodies has become a very important issue in relation to emission gas control for the purpose of preserving global environments. More recently, efforts are made to achieve higher strength of automotive steel sheets and to reduce steel sheet thickness in order to reduce the weights of automobile bodies.
Because most of the body parts of automobiles made of steel sheets are formed by press working, steel sheets used must have excellent press formability. In order to achieve excellent press formability, it is necessary to ensure high ductility. Stretch flanging is frequently applied, so that the steel sheets to be used must have a high hole-expanding ratio. In general, however, a higher strength of steel sheet tends to result in a lower ductility and a lower hole-expanding ratio, thus leading to poor press formability. As a result, there has conventionally been an increasing demand for high-strength steel sheets having high ductility and excellent press formability.
Importance is now placed on safety of an automobile body to protect a driver and passengers upon collision, and for this purpose, steel sheets must have improved impact resistance as a standard of safety upon collision. For the purpose of improving the crashworthiness, a higher strength in a completed automobile is more favorable. There has therefore been the strongest demand for steel sheets having low strength, high ductility, and excellent press formability upon forming automobile parts, and having high strength and excellent crashworthiness in completed products.
To satisfy such a demand, a steel sheet high both in press formability and strength was developed. This is a bake hardenable type steel sheet of which the yield stress increases by applying a bake treatment including holding at a high temperature of 100 to 200xc2x0 C. after press forming. In this steel sheet, the C content remaining finally in a solid solution state (solute C content) is controlled within an appropriate range so as to keep the softness, shape fixability, and ductility during press forming. In a bake treatment performed after the press forming of this steel sheet, the solute C is fixed to a dislocation introduced during the press forming and inhibits the movement of the dislocation, thus resulting in an increase in yield stress. In this bake hardenable type automotive steel sheet, the yield stress can be increased, but the tensile strength cannot be increased.
Japanese Examined Patent Application Publication No. 5-24979 discloses a bake hardenable high-strength cold-rolled steel sheet having a composition comprising C: 0.08 to 0.20%, Mn: 1.5 to 3.5% and the balance Fe and incidental impurities, and having a structure composed of uniform bainite containing not more than 5% of ferrite or composed of bainite partially containing martensite. The cold-rolled steel sheet disclosed in Japanese Examined Patent Publication No. 5-24979 is manufactured by rapidly cooling the steel sheet to a temperature in the range of 400 to 200xc2x0 C. in the cooling step after continuous annealing and then slowly cooling the same. A high degree of baking hardening conventionally unavailable is thereby achieved through conversion from the conventional structure mainly comprising ferrite to a structure mainly comprising bainite in the steel sheet.
In the steel sheet disclosed in Japanese Examined Patent Application Publication No. 5-24979, a high degree of baking hardening conventionally unavailable is obtained through an increase in yield strength after bake treatment. Even in this steel sheet, however, it is yet difficult to increase tensile strength after the bake treatment, and an improvement in crashworthiness cannot still be achieved.
On the other hand, some hot-rolled steel sheets are proposed with a view to increasing not only yield stress but also tensile strength by applying a heat treatment after press forming.
For example, Japanese Examined Patent Application Publication No. 8-23048 proposes a method for manufacturing a hot-rolled steel sheet comprising the steps of reheating a steel containing C: 0.02 to 0.13%, Si: not more than 2.0%, Mn: 0.6 to 2.5%, sol. Al: not more than 0.10%, and N: 0.0080 to 0.0250% to a temperature of not less than 1,100xc2x0 C. and applying hot finish rolling at a temperature of 850 to 950xc2x0 C. The method also comprising the steps of cooling the hot-rolled steel sheet at a cooling rate of not less than 15xc2x0 C./second to a temperature of less than 150xc2x0 C., and coiling the same, thereby forming a composite structure mainly comprising ferrite and martensite. In the steel sheet manufactured by the technique disclosed in Japanese Examined Patent Application Publication No. 8-23048, the tensile strength and the yield stress increase by strain age hardening; however, a serious problem is posed in that coiling of the steel sheet at a very low coiling temperature as less than 150xc2x0 C. results in large variations in mechanical properties. Another problem includes a large variation in increment of yield stress after press forming and bake treatments, as well as poor press formability due to a low hole-expanding ratio (xcex) and decreased stretch-flanging workability.
Japanese Unexamined Patent Application Publication No. 11-199975 proposes a hot-rolled steel sheet for working excellent in fatigue characteristics, containing C: 0.03 to 0.20%, appropriate amounts of Si, Mn, P, S and Al, Cu: 0.2 to 2.0%, and B: 0.0002 to 0.002%, of which the microstructure is a composite structure comprising ferrite as a primary phase and martensite as a second phase, and the ferrite phase contains Cu in a solid-solution and/or precipitation state of not more than 2 nm. The steel sheet disclosed in Japanese Unexamined Patent Application Publication No. 11-199975 has an object based on the fact that the fatigue limit ratio is remarkably improved only when Cu and B are added in combination, and Cu is present in an ultra fine state not more than 2 nm. For this purpose, it is essential to complete hot finish rolling at a temperature above the Ar3 transformation point, air-cool the sheet within the temperature region of Ar3 to Ar1 for 1 to 10 seconds, cool the sheet at a cooling rate of not less than 20xc2x0 C./second, and coil the cooled sheet at a temperature of not more than 350xc2x0 C. A low coiling temperature of not more than 350xc2x0 C. causes serious deformation of the shape of the hot-rolled steel sheet, thus inhibiting industrially stable manufacture.
On the other hand, some automobile parts must have high corrosion resistance. A hot-dip galvanized steel sheet is suitable as a material applied to portions requiring high corrosion resistance. For this reason, a particular demand exists for hot-dip galvanized steel sheets excellent in press formability during forming, and is considerably hardened by a heat treatment after the forming.
To respond to such a demand, for example, Japanese Patent Publication No. 2802513 proposes a method for manufacturing a hot-dip galvanized steel sheet using a hot-rolled steel sheet as a black plate. The method comprises the steps of hot-rolling a steel slab containing C: not more than 0.05%, Mn: 0.05 to 0.5%, Al: not more than 0.1% and Cu: 0.8 to 2.0% at a coiling temperature of not more than 530xc2x0 C. The method further comprising the subsequent steps of reducing the steel sheet surface by heating the hot-rolled steel sheet to a temperature of not more than 530xc2x0 C., and hot-dip-galvanizing the sheet, whereby remarkable hardening is available through a heat treatment after forming. In the steel sheet manufactured by this method, however, the heat treatment temperature must be high as not less than 500xc2x0 C., in order to obtain remarkable hardening from the heat treatment after the forming, and this has a problem in practice.
Japanese Unexamined Patent Application Publication No. 10-310824 proposes a method for manufacturing an alloyed hot-dip galvanized steel sheet having increased strength by a heat treatment after forming, using a hot-rolled or cold-rolled steel sheet as a black plate. This method comprises the steps of hot-rolling a steel containing C: 0.01 to 0.08%, appropriate amounts of Si, Mn, P, S, Al and N, and at least one of Cr, W and Mo: 0.05 to 3.0% in total. The method further comprises the step of cold-rolling or temper-rolling and annealing the sheet. The method still further comprises the step of applying hot-dip galvanizing to the sheet and heating the sheet for alloying treatment. The tensile strength of the steel sheet is increased by heating the sheet at a temperature within the range of 200 to 450xc2x0 C. However, the resultant steel sheet involves a problem in that the microstructure comprises a ferrite single phase, a ferrite and pearlite composite structure, or a ferrite and bainite composite structure; hence, high ductility and low yield strength are unavailable, resulting in low press formability.
The present invention was made in view of the fact that, in spite of the strong demand as described above, a technique for industrially stably manufacturing a steel sheet satisfying these properties has never been found. The present invention solves the problems described above. It is an object of the present invention to provide is directed to high-ductility and high-strength steel sheets suitable for automobiles and having excellent press formability and excellent strain age hardenability, in which the tensile strength increases considerably through a heat treatment at a relatively low temperature after press forming. It is also an object of the present invention to provide a manufacturing method capable of stably manufacturing the high-ductility and high-strength steel sheets.
To achieve the above-mentioned object of the invention, the inventors carried out extensive studies on the effect of the steel sheet structure and alloying elements on strain age hardenability. As a result, the inventors found that a steel sheet having high age hardenability which leads to both an increase in yield stress and a remarkable increase in tensile strength can be obtained after a pre-deformation treatment with a prestrain of not less than 5% and a heat treatment at a relatively low temperature as within the range of 150 to 350xc2x0 C. by (1) forming a composite structure of the steel sheet comprising ferrite and a phase containing retained austenite in a volume ratio of not less than 1%, and (2) limiting the C content within the range of a low-carbon region to a medium-carbon region and containing Cu within an appropriate range or at least one of Mo, Cr, and W in place of Cu. In addition, the steel sheet was found to have satisfactory ductility, a high hole expanding ratio, and excellent press formability.
The results of a fundamental experiment carried out by the inventors on hot-rolled steel sheets will first be described.
A sheet bar having a composition comprising, in weight percent, C: 0.10%, Si: 1.4%, Mn: 1.5%, P: 0.01%, S: 0.005%, Al: 0.04%, N: 0.002% and Cu: 0.3 or 1.3% was heated to 1,250xc2x0 C. and soaked. Then, the sheet bar was subjected to three-pass rolling into a thickness of 2.0 mm so that the finish rolling end temperature was 850xc2x0 C. Thereafter, cooling conditions and the coiling temperature were changed variously to convert a single ferrite structure steel sheet into a hot-rolled steel sheet with a composite structure composed of ferrite as a primary phase and a retained austenite-containing phase as a secondary phase (hereinafter, referred to also as a composite ferrite/retained austenite structure).
Tensile properties were investigated by a tensile test on the resultant hot-rolled steel sheets. A pre-deformation treatment of a tensile prestrain of 5% was applied to each test piece sampled from these hot-rolled steel sheets. Then, after applying a heat treatment at 50 to 350xc2x0 C. for 20 minutes, a tensile test was carried out to determine tensile properties, and the strain age hardenability was evaluated.
The strain age hardenability was evaluated in terms of the increment xcex94TS that is a difference between the tensile strength TSHT after heat treatment and the tensile strength TS before the heat treatment. That is, xcex94TS=(tensile strength TSHT after heat treatment)xe2x88x92(tensile strength TS before pre-deformation treatment). The tensile test was carried out by using JIS No. 5 tensile test pieces sampled in the rolling direction.
FIG. 1 illustrates the effect of the Cu content on the relationship between xcex94TS and the steel sheet structure. A pre-deformation treatment of a tensile prestrain of 5% and then a heat treatment of 250xc2x0 C. for 20 minutes were applied to the test pieces. The increment xcex94TS was determined from the difference in tensile strength TS between before and after the heat treatment. FIG. 1 suggests that, for a Cu content of 1.3 wt. %, a high strain age hardenability as represented by a xcex94TS of not less than 80 MPa is obtained by forming a composite ferrite/retained austenite steel sheet structure. For a Cu content of 0.3 wt. %, xcex94TS is less, than 80 MPa, irrespective of the steel sheet structure, and high strain age hardenability cannot be obtained.
It is possible to manufacture a hot-rolled steel sheet having a high strain age hardenability by limiting the Cu content within an appropriate range, and forming a composite structure having ferrite as a primary phase and a retained austenite-containing phase as a secondary phase.
FIG. 2 illustrates the effect of the Cu content on the relationship between xcex94TS and the heat treatment temperature after pre-strain treatment. The microstructure of the steel sheet is a composite structure having ferrite as a primary phase and a retained austenite-containing phase as a secondary phase, and the volume ratio of the retained austenite structure is 8% of the entire structure.
FIG. 2 shows that the increment xcex94TS increases as the heat treatment temperature increases and strongly depends on the Cu content. With a Cu content of 1.3 wt. %, a high strain age hardenability as represented by a xcex94TS of not less than 80 MPa is obtained at a heat treatment temperature of not less than 150xc2x0 C. For a Cu content of 0.3 wt. %, xcex94TS is less than 80 MPa at any heat treatment temperature, and high strain age hardenability cannot be obtained.
In addition, a hole expanding test was carried out on steel sheets having a single ferrite structure or a composite ferrite/retained austenite structure, and Cu contents of 0.3 wt % and 1.3 wt %, and the hole expanding ratio xcex was determined. In the hole expanding test, punch holes were formed in test pieces through punching with a punch having a diameter of 10 mm. Thereafter, hole expansion was conducted with a conical punch having a vertical angle of 60 degrees so that the burr was outside, until cracks passing through the sheet in the thickness direction form. The hole expanding ratio xcex was determined by the formula: xcex(%)={(dxe2x88x92d0)/d0}xc3x97100 where d0 represents the initial hole diameter, and d represents the hole inside diameter on occurrence of cracks.
In the case of a Cu content of 1.3 wt %, a hot-rolled steel sheet having a composite ferrite/retained austenite structure had a hole expanding ratio of about 140%, and a hot-rolled steel sheet having a single ferrite structure also had a hole expanding ratio of about 140%. In contrast, in the case of a Cu content of 0.3 wt %, a hot-rolled steel sheet having a single ferrite structure had a hole expanding ratio of 120%, and a hot-rolled steel sheet having a composite ferrite/retained austenite structure had a hole expanding ratio of about 80%.
As described above, it is clear that the hot-rolled steel sheet having a composite ferrite/retained austenite structure has an increased hole expanding ratio and that hole expanding formability is improved with an increased Cu content. A detailed mechanism of the improvement in hole expanding formability by Cu has not yet been clarified. The contained Cu is considered to reduce the difference in hardness between the ferrite/retained austenite and the strain-induced transformed martensite.
In the hot-rolled steel sheet of the present invention, very fine Cu precipitates in the steel sheet as a result of a pre-deformation with a strain of 2% or more as measured upon measuring the increment of deformation stress from before to after a usual heat treatment and the heat treatment carried out at a relatively low temperature in the range of 150 to 350xc2x0 C. According to a study carried out by the present inventors, high strain age hardenability bringing about an increase in yield stress and a remarkable increase in tensile strength probably achieved by the precipitation of very fine Cu. Such precipitation of very fine Cu by a heat treatment in a low-temperature region has never been observed in ultra-low carbon steel or low-carbon steel in reports so far released. A reason for precipitation of very fine Cu in a heat treatment at a low temperature has not as yet been clarified to date. However, it is presumable as follows. During isothermal holding in the temperature range of 620 to 780xc2x0 C. or during slow cooling from this temperature range after rapid cooling subsequent to hot rolling, a large amount of Cu is distributed to the xcex3 phase. After cooling, Cu is dissolved in the retained austenite in a supersaturation state. The retained austenite is transformed into martensite by a prestrain of not less than 5%, and very fine Cu precipitates in the strain-induced transformed martensite during a subsequent low-temperature treatment.
Next, the results of a fundamental experiment carried out by the present inventors on the cold-rolled steel sheet will be described.
A sheet bar having a composition comprising, in weight percent, C: 0.10%, Si: 1.2%, Mn: 1.4%, P: 0.01%, S: 0.005%, Al: 0.03%, N: 0.002%, and Cu: 0.3 or 1.3% was heated to 1,250xc2x0 C., soaked and subjected to three-pass rolling into a thickness of 4.0 mm so that the finish rolling end temperature was 900xc2x0 C. After the completion of finish rolling, a temperature holding equivalent treatment of 600xc2x0 C. for 1 hour was applied as a coiling treatment. Thereafter, the sheet was cold-rolled at a reduction of 70% into a cold-rolled steel sheet having a thickness of 1.2 mm. Then, the cold-rolled sheet was heated at a temperature in the range of 700 to 850xc2x0 C. and soaked for 60 seconds. Thereafter, the sheet was cooled to 400xc2x0 C., and was held at the temperature (400xc2x0 C.) for 300 seconds for recrystallization annealing. By the recrystallization annealing, various cold-rolled steel sheets were obtained in which the structure changed from a single ferrite structure to a composite ferrite/retained austenite structure.
Tensile tests were conducted on the resultant cold-roll steel sheets as in the hot-rolled steel sheets to determine tensile properties. Tensile properties (YS, TS) were determined by sampling test pieces from these cold-rolled steel sheets, applying a pre-deformation treatment with a tensile prestrain of 5% to these test pieces, then heating the steel sheets at 50 to 350xc2x0 C. for 20 minutes, and then conducting the tensile tests.
The strain age hardenability was evaluated in terms of the tensile strength increment xcex94TS from before to after the heat treatment, as in the hot-rolled steel sheet.
FIG. 3 illustrates the effect of the Cu content on the relationship between xcex94TS and the recrystallization annealing temperature. The value xcex94TS was determined by applying a pre-deformation treatment with a tensile prestrain of 5% to test pieces sampled from the resultant cold-rolled steel sheets, conducting a heat treatment of 250xc2x0 C. for 20 minutes, and carrying out a tensile test.
FIG. 3 suggests that a high strain age hardenability as represented by a xcex94TS of not less than 80 MPa is available, in the case of a Cu content of 1.3 wt. %, by employing a recrystallization annealing temperature of not less than 750xc2x0 C. to convert the steel sheet structure into a composite ferrite/retained austenite structure. On the other hand, in the case of a Cu content of 0.3 wt. %, high strain age hardenability is unavailable because xcex94TS is less than 80 MPa at any recrystallization annealing temperature. FIG. 3 suggests the possibility of manufacturing a cold-rolled steel sheet having a high strain age hardenability by optimizing the Cu content and forming a composite ferrite/retained austenite structure.
FIG. 4 illustrates the effect of the Cu content on the relationship between xcex94TS and the heat treatment temperature after pre-strain treatment. The steel sheet used was annealed at 800xc2x0 C., which was the dual phase region of ferrite (xcex1)+austenite (xcex3), for a holding time of 60 seconds after cold rolling, cooled from the holding temperature (800xc2x0 C.) to 400xc2x0 C. at a cooling rate of 30xc2x0 C./second, and held at 400xc2x0 C. for 300 seconds. The steel sheets had a composite ferrite/retained austenite (secondary phase) microstructure, the volume ratio of the retained austenite structure being 4%.
FIG. 4 shows that the increment xcex94TS increases as the heat treatment temperature increases and strongly depends on the Cu content. With a Cu content of 1.3 wt. %, a high strain age hardenability as represented by a xcex94TS of not less than 80 MPa is obtained at a heat treatment temperature of not less than 150xc2x0 C. For a Cu content of 0.3 wt. %, xcex94TS is less than 80 MPa at any heat treatment temperature, and high strain age hardenability cannot be obtained.
In addition, a hole expanding test was carried on cold-rolled steel sheets having a composite ferrite/retained austenite structure and Cu contents of 0.3 wt % and 1.3 wt. % to determine the hole expanding ratio (xcex), as in the hot-rolled steel sheet.
In the cold-rolled steel sheet with a Cu content of 1.3%, xcex was 130%; while in the cold-rolled steel sheet with a Cu content of 0.3%, xcex was 60%. It is clear that, for a Cu content of 1.3 wt. %, the hole expanding ratio is increased and hole expanding formability is improved even in the cold-rolled steel sheet, as in the hot-rolled steel sheet. A detailed mechanism of improvement in hole expanding formability with content of Cu has not yet been clarified, as in the hot-rolled steel sheet. Also, in the cold-rolled steel sheet, it is considered that the contained Cu reduces the difference in hardness between the ferrite/retained austenite structure and the strain-induced transformed martensite structure.
In the cold-rolled steel sheet of the present invention, very fine Cu precipitates in the steel sheet as a result of a pre-deformation with a strain larger than 2%, which is equivalent to the prestrain on measuring the deformation stress increment from before to after a usual heat treatment, and a heat treatment at a relatively low temperature of 150 to 350xc2x0 C. According to a study carried out by the present inventors, also in the cold-rolled steel sheet, high strain age hardenability bringing about an increase in yield stress and a remarkable increase in tensile strength is probably achieved by the precipitation of very fine Cu. A reason for precipitation of very fine Cu in a heat treatment in a low temperature region has not as yet been clarified to date. However, it is presumable as follows. During recrystallization annealing in the dual phase region of xcex1+xcex3, a large amount of Cu is distributed to the xcex3 phase. The distributed Cu remains even after cooling and is dissolved into the martensite in a supersaturation state, and very fine Cu precipitates through a prestrain of not less than 5% and a low-temperature treatment.
Next, the result of a fundamental experiment carried out by the present inventors on the hot-dip galvanized steel sheet will be described.
A sheet bar having a composition comprising, in weight percent, C: 0.08%, Si: 0.5%, Mn: 2.0%, P: 0.01%, S: 0.004%, Al: 0.04%, N: 0.002% and Cu: 0.3 or 1.3% was heated to 1,250xc2x0 C. and soaked. Then, the sheet bar was subjected to three-pass rolling into a thickness of 4.0 mm so that the finish rolling end temperature was 900xc2x0 C. After the finish rolling, a temperature holding equivalent treatment of 600xc2x0 C. for 1 h was applied as a coiling treatment. Thereafter, the hot-rolled sheet was cold-rolled at a reduction of 70% into a cold-rolled steel sheet having a thickness of 1.2 mm. Then, the cold-rolled sheet was heated and soaked at 900xc2x0 C., and cooled at a cooling rate of 30xc2x0 C./sec. (a primary heat treatment). The steel sheet after the primary heat treatment had a lath martensite structure. The steel sheet after the primary heat treatment was subjected to a secondary heat treatment at various temperatures, then rapidly cooled to a temperature in the range of 450 to 500xc2x0 C. Then, the sheet was immersed into a hot-dip galvanizing bath (0.13 wt. % Alxe2x80x94Zn bath) to form a hot-dip galvanizing layer on the surface. Further, the sheet was reheated to a temperature in the range of 450 to 550xc2x0 C. to alloy the hot-dip galvanizing layer (Fe content in the galvanizing layer: about 10%).
For the resultant hot-dip galvanized steel sheet, tensile properties were determined through a tensile test. In addition, test pieces were sampled from the hot-dip galvanized steel sheet, and a pre-deformation treatment with a tensile prestrain of 5% was applied to the test pieces, as in the hot-rolled steel sheet and the cold-rolled steel sheet. Then, a heat treatment of 50 to 350xc2x0 C. for 20 minutes was applied. Thereafter, a tensile test was carried out to determine tensile properties. The strain age hardenability was evaluated in terms of the increment xcex94TS of the tensile strength from before to after the heat treatment.
FIG. 5 illustrates the effect of the Cu content on the relationship between xcex94TS and the secondary heat treatment temperature. The increment xcex94TS was determined by applying a tensile prestrain of 5% to test pieces sampled from the resultant hot-dip galvanized steel sheets, conducting a heat treatment at 250xc2x0 C. for 20 minutes, and carrying out a tensile test.
FIG. 5 suggests that, for a Cu content of 1.3 wt. %, a high strain age hardenability as represented by a xcex94TS of not less than 80 MPa can be obtained by forming a composite ferrite/tempered martensite/retained austenite steel sheet structure. In contrast, in the case of a Cu content of 0.3 wt. %, high strain age hardenability cannot be obtained as because xcex94TS is less than 80 MPa at any secondary heat treatment temperature.
FIG. 5 suggests the possibility of manufacturing a hot-dip galvanized steel sheet having high strain age hardenability by optimizing the Cu content and by forming a composite ferrite/tempered martensite/retained austenite structure.
FIG. 6 illustrates the effect of the Cu content on the relationship between xcex94TS and the heat treatment temperature after pre-strain treatment. The increment xcex94TS was determined by applying a tensile prestrain of 5% to test pieces sampled from the alloyed hot-dip galvanized steel sheets treated at a secondary heat treatment temperature of 800xc2x0 C., conducting a heat treatment of 50 to 350xc2x0 C. for 20 minutes, and carrying out a tensile test.
FIG. 6 shows that the increment xcex94TS increases as the heat treatment temperature increases after the pre-deformation treatment and strongly depends on the Cu content. With a Cu content of 1.3 wt. %, a high strain age hardenability as represented by a xcex94TS of not less than 80 MPa can be obtained at a heat treatment temperature of not less than 150xc2x0 C. In contrast, for a Cu content of 0.3 wt. %, xcex94TS is less than 80 MPa at any heat treatment temperature, and high strain age hardenability cannot be obtained.
In the hot-dip galvanized steel sheet of the present invention, very fine Cu precipitates in the steel sheet as a result of a pre-deformation with a strain larger than 2% which is a usual amount of strain on measuring the deformation stress increment from before to after a heat treatment, and a heat treatment within a relatively low temperature region of 150 to 350xc2x0 C. According to a study carried out by the present inventors, high strain age hardenability bringing about an increase in yield stress and a remarkable increase in tensile strength is probably achieved by the precipitation of very fine Cu. A reason for precipitation of very fine Cu in a heat treatment in a low temperature region has not as yet been clarified to date. However, it is presumable as follows. During heat treatment in the dual phase region of ferrite (xcex1)+austenite (xcex3), a large amount of Cu is distributed to the xcex3 phase, and the distributed Cu remaining even after cooling is dissolved into the retained austenite in a supersaturation state. The retained austenite is transformed into martensite by a prestrain of not less than 5%, and very fine Cu precipitates in the martensite through a subsequent low-temperature heat treatment.
In addition, hole expanding test was performed using hot-dip galvanized steel sheets having a composite structure of ferrite/tempered martensite/retained austenite and Cu contents of 0.3 wt % and 1.3 wt. % to determine the hole expanding ratio (xcex), as in the hot-rolled steel sheet and the cold-rolled steel sheet.
The hole expanding ratio xcex of the steel sheet having a Cu content of 1.3% was 120%, while the hole expanding ratio xcex of the steel sheet having a Cu content of 0.3% was 50%. The results suggest that for a Cu content of 1.3 wt %, the hole expanding ratio is increased and hole expanding formability is improved, as compared with a Cu content of 0.3%.
A detailed mechanism of improvement in hole expanding formability with content of Cu has not yet been clarified, as in the hot-rolled steel sheet and the cold-rolled steel sheet, but it is considered that the contained Cu reduces the difference in hardness among the ferrite, the tempered martensite/retained austenite, and the martensite formed by strain induced transformation.
On the basis of the novel findings as described above, the present inventors carried out further extensive studies and found that the above-mentioned phenomena occurred in steel sheets not containing Cu as well.
The structure of a steel sheet having a composition containing at least one of Mo, Cr, and W was converted to a composite structure containing a ferrite primary phase and a phase containing retained austenite as a secondary phase. Thereafter, by applying a prestrain and a heat treatment in a low temperature region, it was found that very fine carbides precipitated in the strain-induced transformed martensite, resulting in an increase in tensile strength. The strain-induced fine precipitation at a low temperature was more remarkable in a steel composition containing at least one of Nb, Ti, and V in addition to at least one of Mo, Cr, and W.
The present invention was completed through further studies on the basis of the aforementioned findings. The gist of the present invention is as follows:
(1) A high-ductility steel sheet excellent in press formability and in strain age hardenability as represented by a xcex94TS of not less than 80 MPa, comprising a composite structure containing a primary phase containing a ferrite phase and a secondary phase containing a retained austenite phase in a volume ratio of not less than 1%.
(2) A high-ductility steel sheet according to aspect (1), wherein the steel sheet is a hot-rolled steel sheet, and the primary phase consisting essentially of a ferrite phase.
(3) A high-ductility steel sheet according to aspect (2), wherein the hot-rolled steel sheet has a composition comprising, in weight percent, C: 0.05 to 0.20%, Si: 1.0 to 3.0%, Mn: not more than 3.0%, P: not more than 0.10%, S: not more than 0.02%, Al: not more than 0.30%, N: not more than 0.02%, and Cu: 0.5 to 3.0%, and the balance Fe and incidental impurities.
(4) A high-ductility steel sheet according to aspect (3), the composition further comprising, in weight percent, at least one of the following Groups A to C:
Group A: Ni: not more than 2.0%;
Group B: at least one of Cr and Mo: not more than 2.0% in total; and
Group C: at least one of Nb, Ti, and V: not more than 0.2% in total.
(5) A high-ductility steel sheet according to aspect (2), wherein the hot-rolled steel sheet has a composition comprising, in weight percent, C: 0.05 to 0.20%, Si: 1.0 to 3.0%, Mn: not more than 3.0%, P: not more than 0.10%, S: not more than 0.02%, Al: not more than 0.30%, N: not more than 0.02%, at least one of Mo: 0.05 to 2.0%, Cr: 0.05 to 2.0% and W: 0.05 to 2.0%, not more than 2.0% in total, and the balance Fe and incidental impurities.
(6) A high-ductility steel sheet according to aspect (5), the composition further containing, in weight percent, at least one of Nb, Ti, and V in an amount of not more than 2.0% in total.
(7) A method for manufacturing a high-ductility hot-rolled steel sheet excellent in press formability and in strain age hardenability as represented by a xcex94TS of not less than 80 MPa, comprising the steps of: hot-rolling a steel slab having a composition comprising, in weight percent, C: not more than 0.20%, Si: 1.0 to 3.0%, Mn: not more than 3.0%, P: not more than 0.10%, S: not more than 0.02%, Al: not more than 0.30%, N: not more than 0.02%, and Cu: 0.5 to 3.0%, into a hot-rolled steel sheet having a prescribed thickness, the hot rolling step including finish-rolling at a finish rolling end temperature of 780 to 980xc2x0 C.; cooling the finish-rolled steel sheet to a temperature in the range of 620 to 780xc2x0 C. within 2 seconds at a cooling rate of at least 50xc2x0 C./second; holding the sheet at the temperature in the range of 620 to 780xc2x0 C. for 1 to 10 seconds, or slowly cooling the sheet at a cooling rate of not more than 20xc2x0 C./second; cooling the sheet at a cooling rate of not less than 50xc2x0 C./second to a temperature of 300 to 500xc2x0 C.; and coiling the sheet.
(8) A method for manufacturing a high-ductility hot-rolled steel sheet excellent in press formability and in strain age hardenability as represented by a xcex94TS of at least 80 MPa, according to aspect (7), the composition further comprising, in weight percent, at least one of the following Groups A to C:
Group A: Ni: not more than 2.0%;
Group B: at least one of Cr and Mo: not more than 2.0% in total; and
Group C: at least one of Nb, Ti, and V: not more than 0.2% in total.
(9) A method for manufacturing a high-ductility hot-rolled steel sheet according to aspect (7), wherein the steel slab is replaced with a steel slab having a composition containing, in weight percent, C: 0.05 to 0.20%, Si: 1.0 to 3.0%, Mn: not more than 3.0%, P: not more than 0.10%, S: not more than 0.02%, Al: not more than 0.30%, N: not more than 0.02%, and at least one of Mo: 0.05 to 2.0%, Cr: 0.05 to 2.0% and W: 0.05 to 2.0% in a total amount of not more than 2.0%.
(10) A method for manufacturing a high-ductility hot-rolled steel sheet according to aspect (9), the composition further containing, in weight percent, at least one of Nb, Ti, and V in a total amount of not more than 2.0%.
(11) A method for manufacturing a high-ductility hot-rolled steel sheet according to any one of aspects (7) to (10), wherein all or part of the finish rolling is lubrication rolling.
(12) A high-ductility steel sheet according to aspect (1), wherein the steel sheet is a cold-rolled steel sheet, and the primary phase containing the ferrite phase is a ferrite phase.
(13) A high-ductility steel sheet according to aspect (12), wherein the cold-rolled steel sheet has a composition comprising, in weight percent, C: not more than 0.20%, Si: not more than 2.0%, Mn: not more than 3.0%, P: not more than 0.1%, S: not more than 0.02%, Al: not more than 0.3%, N: not more than 0.02%, Cu: 0.5 to 3.0%, and the balance Fe and incidental impurities.
(14) A high-ductility steel sheet according to aspect (13), the composition further comprising, in weight percent, at least one of the following Groups A to C:
Group A: Ni: not more than 2.0%;
Group B: at least one of Cr and Mo: not more than 2.0% in total; and
Group C: at least one of Nb, Ti, and V: not more than 0.2% in total.
(15) A high-ductility steel sheet according to aspect (12), wherein the cold-rolled steel sheet has a composition comprising, in weight percent: C: not more than 0.20%, Si: not more than 2.0%, Mn: not more than 3.0%, P: not more than 0.1%, S: not more than 0.02%, Al: not more than 0.3%, N: not more than 0.02%, at least one selected from the group consisting of Mo: 0.05 to 2.0%, Cr: 0.05 to 2.0% and W: 0.05 to 2.0%, not more than 2.0% in total, and the balance Fe and incidental impurities.
(16) A high-ductility steel sheet according to aspect (15), the composition further comprising, in weight percent, at least one of Nb, Ti, and V, in a total amount of not more than 2.0%.
(17) A method for manufacturing a high-ductility cold-rolled steel sheet excellent in press formability and in strain age hardenability as represented by a xcex94TS of not less than 80 MPa, comprising: a hot rolling step of hot-rolling a steel slab having a composition containing, in weight percent, C: not more than 0.20%, Si: not more than 2.0%, Mn: not more than 3.0%, P: not more than 0.1%, S: not more than 0.02%, Al: not more than 0.3%, N: not more than 0.02%, and Cu: 0.5 to 3.0% as a material to form a hot-rolled steel sheet; a cold rolling step of cold-rolling the hot-rolled steel sheet into a cold-rolled steel sheet; and a recrystallization annealing step of applying recrystallization annealing to the cold-rolled steel sheet into a cold-rolled annealed steel sheet, the recrystallization annealing step including a heat treatment of heating and soaking the steel sheet in a ferrite/austenite dual phase region within a temperature range of the AC1 transformation point to the AC3 transformation point, cooling the sheet, and holding the sheet in the temperature region of 300 to 500xc2x0 C. for 30 to 1,200 seconds.
(18) A method for manufacturing a high-ductility cold-rolled steel sheet according to aspect (17), the composition further containing, in weight percent, at least one selected from the following Groups A to C:
Group A: Ni: not more than 2.0%;
Group B: at least one of Cr and Mo: not more than 2.0% in total; and
Group C: at least one of Nb, Ti, and V: not more than 0.2% in total.
(19) A method for manufacturing a high-ductility cold-rolled steel sheet according to aspect (17), wherein the steel slab is replaced with a steel slab having a composition containing, in weight percent, C: not more than 0.20%, Si: not more than 2.0%, Mn: not more than 3.0%, P: not more than 0.10%, S: not more than 0.02%, Al: not more than 0.3%, N: not more than 0.02%, and at least one selected from the group consisting of Mo: 0.05 to 2.0%, Cr: 0.05 to 2.0% and W: 0.05 to 2.0% in a total amount of not more than 2.0%.
(20) A method of manufacturing a high-ductility cold-rolled steel sheet according to aspect (19), the composition further containing, in weight percent, at least one of Nb, Ti, and V in a total amount of not more than 2.0%.
(21) A method for manufacturing a high-ductility cold-rolled steel sheet according to any one of aspects (17) to (20), wherein the hot-rolling step includes heating the steel slab at a temperature of not less than 900xc2x0 C., rolling the slab at a finish rolling end temperature of not less than 700xc2x0 C., and coiling the hot-rolled steel sheet at a coiling temperature of not more than 800xc2x0 C.
(22) A method for manufacturing a cold-rolled steel sheet according to any one of aspects (17) to (21), wherein all or part of the hot rolling is lubrication rolling.
(23) A high-ductility hot-dip galvanized steel sheet comprising a hot-dip galvanizing layer or an alloyed hot-dip galvanizing layer formed on the surface of the high-ductility steel sheet according to any one of aspects (1) to (6).
(24) A high-ductility hot-dip galvanized steel sheet comprising a hot-dip galvanizing layer or an alloyed hot-dip galvanizing layer formed on the surface of the high-ductility steel sheet according to any one of aspects (12) to (16).
(25) A high-ductility steel sheet according to aspect (1), wherein the steel sheet is a hot-dip galvanized steel sheet having a hot-dip galvanizing layer or an alloyed hot-dip galvanizing layer formed on a surface of the steel sheet, and the primary phase containing a ferrite phase comprises a ferrite phase and a tempered martensite phase.
(26) A high-ductility steel sheet according to aspect (25), wherein the steel sheet has a composition comprising, in weight percent, C: not more than 0.20%, Si: not more than 2.0%, Mn: not more than 3.0%, P: not more than 0.1%, S: not more than 0.02%, Al: not more than 0.3%, N: not more than 0.02%, Cu: 0.5 to 3.0%, and the balance Fe and incidental impurities.
(27) A high-ductility steel sheet according to aspect (26), the composition further containing, in weight percent, at least one of the following Groups A to C:
Group A: Ni: not more than 2.0%;
Group B: at least one of Cr and Mo: not more than 2.0% in total; and
Group C: at least one of Nb, Ti, and V: not more than 0.2% in total.
(28) A high-ductility steel sheet according to aspect (25), wherein the steel sheet has a composition comprising, in weight percent, C: not more than 0.20%, Si: not more than 2.0%, Mn: not more than 3.0%, P: not more than 0.1%, S: not more than 0.02%, Al: not more than 0.3%, N: not more than 0.02%, at least one selected from the group consisting of Mo: 0.05 to 2.0%, Cr: 0.05 to 2.0% and W: 0.05 to 2.0% in a total amount of not more than 2.0%, and the balance Fe and incidental impurities.
(29) A high-ductility steel sheet according to aspect (28), the composition further containing, in weight percent, at least one of Nb, Ti, and V in a total amount of not more than 2.0%.
(30) A method for manufacturing of a high-ductility hot-dip galvanized steel sheet excellent in press formability and in strain age hardenability as represented by a xcex94TS of not less than 80 MPa, comprising: a primary heat-treating step of heating a steel sheet to a temperature of not less than the AC1 transformation point and rapidly cooling the steel sheet, the steel sheet having a composition containing, in weight percent, C: not more than 0.20%, Si: not more than 2.0%, Mn: not more than 3.0%, P: not more than 0.1%, S: not more than 0.02%, Al: not more than 0.3%, N: not more than 0.02%, and Cu: 0.5 to 3.0%; a secondary heat-treating step of heating the steel sheet to a temperature in the range of the AC1 transformation point to the AC3 transformation point; and a hot-dip galvanizing step of forming a hot-dip galvanizing layer on the surface of the steel sheet.
(31) A method for manufacturing a high-ductility cold-rolled steel sheet according to aspect (30), the composition further containing, in weight percent, at least one of the following Groups A to C:
Group A: Ni: not more than 2.0%;
Group B: at least one of Cr and Mo: not more than 2.0% in total; and
Group C: at least one of Nb, Ti, and V: not more than 0.2% in total.
(32) A method for manufacturing a high-ductility hot-dip galvanized steel according to aspect (30), wherein the steel sheet is replaced with a steel sheet having a composition comprising, in weight percent, C: not more than 0.20%, Si: not more than 2.0%, Mn: not more than 3.0%, P: not more than 0.1%, S: not more than 0.02%, Al: not more than 0.3%, N: not more than 0.02%, and at least one selected from the group consisting of Mo: 0.05 to 2.0%, Cr: 0.05 to 2.0% and W: 0.05 to 2.0% in a total amount of not more than 2.0%.
(33) A method for manufacturing a high-ductility hot-dip galvanized steel sheet according to aspect (32), the composition further containing, in weight percent, at least one of Nb, Ti, and V in a total amount of not more than 2.0%.
(34) A method for manufacturing a high-ductility hot-dip galvanized steel sheet according to any one of aspects (30) to (33), further comprising a pickling treatment step of pickling the steel sheet between the primary heat-treating step and the secondary heat-treating step.
(35) A method for manufacturing a high-ductility hot-dip galvanized steel sheet according to any one of aspects (30) to (34), further comprising an alloying step of alloying the hot-dip galvanizing layer, subsequent to the hot-dip galvanizing step.
(36) A method for manufacturing a high-strength hot-dip galvanized steel sheet according to any one of aspects (30) to (35), wherein the steel sheet is a hot rolled steel sheet manufactured by hot-rolling a material under conditions including a heating temperature of not less than 900xc2x0 C., a finish rolling end temperature of not less than 700xc2x0 C. and a coiling temperature of not more than 800xc2x0 C., or a cold-rolled steel sheet obtained by cold-rolling the hot-rolled steel sheet.
(37) A method for manufacturing a high-strength hot-dip galvanized steel sheet according to aspect (36), wherein the cold-rolling is performed at a reduction ratio of not less than 40%.